Hydraulic valve lash adjusters serve to adjust the lash that is formed between the transmission elements during the transmission of the cam lift by a camshaft to a gas exchange valve of the internal combustion engine due to wear or thermal expansion. Lash adjusters are used with the aim of achieving a low-noise and low-wear operation of the valve train and the greatest possible compliance between the cam lobe and the lift of the respective gas exchange valve.
Prior art last adjusters of the pre-cited type possess a control valve in the form of a one-way valve that comprises a closing body, for example, a ball. The closing body may be loaded by a spring of the control valve. In the case of a switchable lash adjuster with a control valve disclosed in the document DE 102 04 673 A1, the closing body is loaded by a control valve spring in closing direction. As a result, the control valve is mostly closed and an idle stroke of the lash adjuster does not take place. However, with this configuration, the danger of a pumping-up of the lash adjuster exists.
This drawback is avoided with control valves whose control valve springs load the closing body in the opening direction, or control valves that have no spring at all. Due to the reversed arrangement of the control valve spring, lash adjusters with such a control valve are called reverse-spring adjusters, or if the spring is omitted, free-ball adjusters. These have a positive effect on the thermodynamics, the pollutant emission and the mechanical loading of the internal combustion engine and are therefore finding increasing use.
In the aforesaid prior art structure, the control valve is closed most of the time in the base circle phase of the cam due to the spring force of the control valve spring. In contrast, in a reverse-spring adjuster, the control valve is kept open in this phase due to the force of the control valve spring, or, in the case of a free-ball adjuster, closing is not forced. Because such an adjuster can only be closed by the hydrodynamic and hydrostatic forces of the lubricating oil stream that starts to flow from the high pressure chamber to the low pressure chamber at the beginning of the cam lobe phase, this adjuster always executes an idle stroke before the valve lift of the gas exchange valve starts. The magnitude of the idle strokes at every engine speed depends on the duration of the closing time of the control valve and this, in turn, depends on the viscosity/density of the lubricating oil that is used here, as is generally known, as a hydraulic medium.
For closing the control valve of a reverse-spring adjuster or a free-ball adjuster, a so-called critical lubricating oil speed is required. This depends on the viscosity of the lubricating oil and thus on the temperature of the lubricating oil. With a high lubricating oil viscosity/density, i.e., at a low lubricating oil temperature, the critical lubricating oil speed is lower and is therefore reached faster than with a lower lubricating oil viscosity, that is to say, higher lubricating oil temperature. At cold start, this leads to a shorter closing time of the control valve and, consequently, to a smaller idle stroke than when the engine is at running temperature. But a smaller idle stroke means a larger valve overlap. This results in a stronger internal recirculation of exhaust gas that causes an erratic low-speed idling of the engine. This can naturally be ameliorated by increasing the idling speed but would be at the expense of pollutant emission and fuel consumption. Non-switchable reverse-spring adjusters are known, for instance, from the documents EP 1 298 287 A2, JP 61-185607 and U.S. Pat. No. 4,054,109. All these show lash adjusters in which the control valve possesses a closing body in the form of a ball. In these prior art examples, the closing body is guided in bores.
As already mentioned, in the case of reverse-spring adjusters and free-ball adjusters, the closing body of the control valve is the opening position during the base circle of the cam. For closing the control valve, a volume stream that causes a differential pressure on the closing body must flow past the closing body. Due to this pressure difference, the closing body closes the control valve. The timing gears of engines with cylinder valve lift reversal or deactivation comprise both switchable and non-switchable lash adjusters. For a better distinction over switchable lash adjusters, the non-switchable lash adjusters are designated as standard lash adjusters in the present context. In order to provide receptions of the same size for all the lash adjusters in the engine housing, due to the different structures of standard lash adjusters and switchable lash adjusters, different dimensions have to be implemented in their piston and control valve regions. Thus, the piston that is guided for axial displacement in the adjuster housing often has a smaller diameter in switchable lash adjusters than in standard lash adjusters. In the case of reverse-spring adjusters, this can lead to different modes of operation, i.e., to variable idle strokes with the corresponding detrimental effects on the thermodynamics of the engine.
In the mode of operation of reverse-spring adjusters and free-ball adjusters, a desired idle stroke of the adjuster housing, i.e., an axial displacement of the housing relative to the non-displaced piston guided therein, is obtained during the idle stroke. Variations of this idle stroke in a single adjuster should be as small as possible. Further, variations of the idle strokes of all the reverse-spring adjusters and free-ball adjusters arranged in a single engine should likewise be as small as possible. If reverse-spring adjusters or free-ball adjusters with different adjuster diameters, i.e. piston diameters, are installed in a single engine, the idle strokes of these adjusters must b matched to one another. If not, detrimental effects on the mode of functioning and on the idle and smooth running of the engine are the result.